1. Field of the Invention
The invention relates to spread spectrum communication, particularly with respect to Global Positioning System (GPS) receivers.
2. Description of the Prior Art
The GPS is a navigation system utilizing a plurality of satellites in diverse orbital positions. Each satellite transmits an L-band carrier biphase modulated by a spectrum spreading Pseudo Random Noise (PRN) code identifying the satellite and by a 50 baud navigation data message. In the present day system, the carrier frequency is 1575.42 MHz and the PRN code has a clock rate of 1.023 MHz and a code length of 1023 bits. The code repeats the predefined sequence thereof once each millisecond (1 Khz repetition rate).
In order to receive the GPS signal structure, a GPS receiver generates an exact duplicate of the spreading code and aligns it in time to the receive code using correlation techniques. The receiver thus removes the spreading code from the carrier (unmodulates the carrier) leaving only the biphase 50 baud data message thereon. The data message is used by the GPS receiving equipment to solve the navigation problem.
Two receiver architectures are generally utilized for GPS; viz, the Long Loop receiver and the Delay Lock Loop (DLL) receiver. These two GPS receiver architectures utilize different methods in maintaining the PRN code in time alignment. In the DLL receiver, the code is maintained aligned by a tracking loop. The tracking loop delays and advances the code in time and generates decisions as to where the best alignment occurs. The Delay Lock Loop must operate continuously and is subject to acquisition anomalies and mistracking caused by noise. The implementation of a code tracking DLL is complex and expensive and requires both digital hardware and software.
The Long Loop Receiver does not require a tracking loop to maintain the code aligned. The Long Loop Receiver phase locks the oscillators thereof to the received carrier and utilizes the oscillators to generate the local PRN code. Since the receiver oscillators are phase locked to the received carrier, they are phase coherent with respect to the satellite transmitter oscillators that generated the transmitted PRN code. Thus, the locally generated PRN code will not drift relative to the received code and therefore no tracking loop is required.
Although the Long Loop Receiver satisfies the desideratum of eliminating the requirement for a code tracking loop, it suffers from a problem associated with such receivers. The sum of the local oscillator frequencies is always equal to the frequency of the receive signal. This locally generated receive frequency tends to leak into the input of the receiver and be down converted through the intermediate frequency (IF) stages. Frequently, a Long Loop Receiver will suffer from false lock-ups where it locks to itself. Poor sensitivity is another symptom of internal leakage. In order to attempt to obviate the leakage problem, critical designs with respect to shielding and physical placement of the receiver components must be utilized. Thus, when Long Loop Receivers are implemented, a large number of carefully packaged shielded enclosures are required to achieve sufficient isolation. For this reason, GPS receivers generally are constructed utilizing Delay Lock Loop techniques with the attendant disadvantages discussed above.
Present day GPS receivers frequently utilize digital data processing after the receiver IF sections for performing phase detection and loop error signal generation to maintain the receiver aligned to the incoming code. Typically, the input signals to the digital section of such receivers are in the megahertz range requiring critical and expensive high speed digital circuitry.